The evaluation of the state of traffic signs installed on the road has normally been done by means of analysing two parameters:                The back-reflection coefficient: Property of back-reflecting materials, which permits light to be returned in the same direction as the incident light.        The luminance: Quantity of light returned by the sign to which the eye is sensitive.        
The back-reflection coefficient is a parameter characteristic of the material, whose value falls off as the sign deteriorates. In order to evaluate it, systems have been developed such as those described in documents U.S. Pat. No. 7,173,707, U.S. Pat. No. 7,043,057 and U.S. Pat. No. 6,674,878, in which procedures are described for measuring the multiple back-reflection values or three-dimensional map of back-reflection coefficients provided by the sign. This measurement is made via a system that is capable of providing illumination by means of a high intensity stroboscopic light and which measures the different values of light intensity. It then generates some surfaces of back-reflection coefficients and simulates the back-reflection values along the road, recreating a virtual drive.
But, although the back-reflection coefficient or the three-dimensional map of back-reflection coefficients is a characteristic parameter of the sign, in order to obtain the luminance value (the parameter that really defines the visibility conditions) indirect methods have to be applied in which theoretical models of the illumination are used. The stated documents therefore lead to the problem of how to make a direct measurement of the luminance without having to be subject to intermediate operations based on unreal theoretical models.
Another problem that is raised by using the back-reflection coefficient lies in the fact that it is only applicable to back-reflecting signs, and signs that are internally lit have to be discarded from the analysis.
To make a direct calculation of the luminance (in units of cd/m2) there exist various apparatus known as luminance meters. In order to make a measurement of the magnitude, these devices require certain specific conditions of stability and illumination, and the measurement they provide is of a point source. This equipment therefore displays serious drawbacks for making a simultaneous measurement while moving of all the signs lying within the analysis zone. The luminance measurements would have to be made one at a time, and in different zones of the sign, in order to then obtain an average of them.
Moreover, as the measurement requires a strictly controlled type of illumination, we would have to discard data that is influenced by the effects of moving traffic, external lighting, etc.
As a consequence, it is an objective of the present invention to have a procedure and a device that would permit the luminance of the sign to be determined:                Directly, making a direct measurement of the physical parameter.        Automatically, for all signs appearing in a scene.        While circulating with a vehicle.        On the basis of a known or standardized light source, independently of the existence of other external light sources at the moment of the measurement.        Independently of whether the sign is back-reflecting, or internally lit, or any other kind of traffic sign.        Independently of the level of back-reflection which this type of sign provides once installed on the road.        